Silicon nitride circuit board and electronic component module using the same

ABSTRACT

The present invention provides a silicon nitride circuit board in which metal plates are attached on front and rear sides of a silicon nitride substrate having a three-point flexural strength of 500 MPa or higher, wherein assuming that a thickness of the metal plate on the front side is denoted by t1, and a thickness of the metal plate on the rear side is denoted by t2, a numerical relation: |t1−t2|≥0.30 mm is satisfied, and a warp is formed in the silicon nitride substrate so that the silicon nitride substrate is convex toward the metal plate on one of the front side or the rear side; and warp amounts of the silicon nitride substrate in a long-side direction and a short-side direction both fall within a range from 0.01 to 1.0 mm. It is preferable that a longitudinal width (L1) of the silicon nitride substrate falls within a range from 10 to 200 mm, and a transverse width (L2) of the silicon nitride substrate falls within a range from 10 to 200 mm. Due to above structure, even if the silicon nitride circuit board has a large difference in thickness between the metal plates attached on front and rear sides of the silicon nitride substrate, TCT properties can be greatly improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/548,035 filed Aug. 1, 2017 (now U.S. Pat. No. 10,109,555, issued Oct.23, 2018), which is a U.S. National Phase Application under 35 U.S.C.371 of International Application No. PCT/JP2016/052190 filed on Jan. 26,2016 and published in Japanese as WO 2016/125635 on Aug. 11, 2016, andwhich claims the benefit of priority from Japanese Patent ApplicationNo. 2015-018515 filed Feb. 2, 2015. The entire disclosures of all of theabove applications are incorporated herein by reference.

DESCRIPTION Technical Field

An embodiment described herein generally relates to a silicon nitridecircuit board and an electronic component module using the siliconnitride circuit board.

Background Art

In recent years, development of high power modules such as high-powerhigh-efficiency inverters has been underway with an increase inperformance of industrial equipment such as robots and motors. Inproportion to outputs of high power modules, heat generated fromsemiconductor elements is steadily increasing. To dissipate (radiate)the heat efficiently, various methods are adopted for high powermodules. Recently, use has been made of a ceramic circuit boardincluding metal plates attached and bonded on both sides of a ceramicsubstrate.

As the ceramic circuit board, the circuit boards described in WO2007/105361 (Patent Document 1), Japanese Patent Laid-Open No.2010-118682 (Patent Document 2) are developed. According to PatentDocument 1, an improved TCT property (thermal cycling resistanceproperty; thermal cycle test property) is given by performing control ofa thickness ratio between a metal plate on a front side (first metalplate) and a metal plate on a rear side (second metal plate) to 50% orlarger and 200% or smaller.

According to Patent Document 2, a ceramic circuit board that warps so asto be convex toward a metal plate on a front side (on a metal circuitplate side) is developed. According to Patent Document 2, by adoptingsuch a structure, an improved solder flow property is given. By adoptingthe structures of the ceramic circuit boards as described in PatentDocument 1 and Patent Document 2, a TCT property and a solder flowproperty are improved.

In accordance with an increase in power of semiconductor elements, afurther improvement of a heat dissipation property has been demanded. Ina semiconductor module, a semiconductor element is mounted on a ceramiccircuit board through a solder layer interposed therebetween. In such amodule structure, a heat transfer path is as follows: semiconductorelement→solder layer→metal plate (metal plate on the front side)→ceramicsubstrate→metal plate (metal plate on the rear side). One of indexesindicating a heat dissipation property of a ceramic circuit board isthermal resistance. A lower thermal resistance indicates a better heatdissipation property.

A thermal resistance (Rth) is determined as Rth=H/(k×A). Here, a symbolH represents heat transfer path, k represents thermal conductivity, andA represents heat dissipation area. As is clear from the abovecalculation expression, decreasing the thermal resistance (Rth) involvesshortening the heat transfer path (H), increasing the thermalconductivity (k), and increasing the heat dissipation area (A).

Moreover, improvement of a TCT (thermal cycle test) property is alsodemanded for ceramic circuit boards. According to Japanese Patent No.3797905 (Patent Document 3), a silicon nitride substrate having athree-point flexural strength of 500 MPa or higher is developed. Use ofa silicon nitride substrate having a high flexural strength (bendingstrength) allows the improvement of the TCT property.

To cope with the increase in power mentioned before, further improvementof the heat dissipation property and the TCT property have beendemanded. To improve the heat dissipation property by using ahigh-strength silicon nitride substrate, it is effective to increase theheat dissipation area (A). To shorten the heat transfer path (H), it iseffective to reduce thicknesses of a metal circuit plate and a ceramicsubstrate.

However, an excessive reduction of the thickness of a metal circuitplate makes it difficult to flow large current. An excessive reductionof the thickness of a ceramic substrate causes concern about aninsulation property. As to the thermal conductivity (k), an aluminumnitride substrate having a thermal conductivity to a degree of 250 W/m·Kis developed. However, such the aluminum nitride substrate has astrength of only about 250 MPa and thus is poor in TCT property.

Examples of effective methods to increase the heat dissipation area (A)include a method to enlarge a metal plate to be attached to a ceramicsubstrate, and a method to attach a leadframe, a heat sink, and thelike.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: International Publication No. WO 2007/105361

Patent Document 2: Japanese Patent Laid-Open No. 2010-118682

Patent Document 3: Japanese Patent No. 3797905

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

According to [Table 1] of Patent Document 1, while the thickness ratiobetween the metal plates on the front side and the rear side is changedto a range from 50 to 250%, a silicon nitride circuit board having agood TCT property is not obtained when a difference in thickness betweenthe front and rear metal plates becomes large, being 0.30 mm or larger.According to [Table 1] of Patent Document 2, a solder flow property isimproved by controlling a warp amount of a silicon nitride substrate ina long-side direction. However, Patent Document 2 provides no sufficientstudy about a case where the difference in thickness between the frontand rear metal plates becomes large, being 0.30 mm or larger.

Furthermore, a conventional silicon nitride circuit board involves aproblem in that the silicon nitride circuit board bends when a leadframeor the like having a thickness of 0.4 mm or larger is attached toincrease the heat dissipation area (A).

An objective to be achieved by the present invention is to provide asilicon nitride circuit board that is good in TCT property even whenmetal plates are attached on front and rear sides thereof, a differencein thickness between the metal plates being 0.30 mm or larger. Inaddition, the present invention provides a silicon nitride circuit boardthat does not bend even when a leadframe or the like having a thicknessof 0.4 mm or larger are attached thereto.

Means for Solving the Problems

The silicon nitride circuit board according to an embodiment is asilicon nitride circuit board in which metal plates are attached onfront and rear sides of a silicon nitride substrate having a three-pointflexural strength of 500 MPa or higher, wherein assuming that athickness of the metal plate on the front side is denoted by t1, and athickness of the metal plate on the rear side is denoted by t2,|t1−t2|≥0.30 mm is satisfied, the silicon nitride substrate warps so asto be convex toward the metal plate on one of the front side or the rearside, and warp amounts of the silicon nitride substrate in a long-sidedirection and a short-side direction both fall within a range from 0.01to 1.0 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view illustrating an example of a silicon nitridecircuit board according to an embodiment.

FIG. 2 is a lateral cross sectional view illustrating an example of thesilicon nitride circuit board according to the embodiment.

FIG. 3 is a top view illustrating an example of a length of a diagonalline of the silicon nitride circuit board according to the embodiment.

FIG. 4 is a lateral cross sectional view illustrating another example ofthe silicon nitride circuit board according to the embodiment.

FIG. 5 is a lateral cross sectional view illustrating yet anotherexample of the silicon nitride circuit board according to theembodiment.

FIG. 6 is a top view illustrating yet another example of the siliconnitride circuit board according to the embodiment.

FIG. 7 is a lateral cross sectional view illustrating yet anotherexample of the silicon nitride circuit board according to theembodiment.

FIG. 8 is a cross-sectional view illustrating an example of a sidesurface of a metal plate of the silicon nitride circuit board accordingto the embodiment.

FIG. 9 is a lateral cross sectional view illustrating an example of anelectronic component module according to the embodiment.

FIG. 10 is a lateral cross sectional view illustrating another exampleof the electronic component module according to the embodiment.

FIG. 11 is a lateral cross sectional view illustrating yet anotherexample of the electronic component module according to the embodiment.

DESCRIPTION OF EMBODIMENT

The silicon nitride circuit board according to an embodiment is asilicon nitride circuit board in which metal plates are attached onfront and rear sides of a silicon nitride substrate having a three-pointflexural strength of 500 MPa or higher, wherein assuming that athickness of the metal plate on the front side is denoted by t1, and athickness of the metal plate on the rear side is denoted by t2, anumerical relation: |t1−t2|≥0.30 mm is satisfied, a warp is formed inthe silicon nitride substrate so that the silicon nitride substrate isconvex toward the metal plate on one of the front side or the rear side,and warp amounts of the silicon nitride substrate in a long-sidedirection and a short-side direction both fall within a range from 0.01to 1.0 mm.

The silicon nitride substrate is one having a three-point flexuralstrength (three-point bending strength) of 500 MPa or higher. When thethree-point flexural strength is as low as less than 500 MPa, a TCTproperty of the silicon nitride substrate decreases. The three-pointflexural strength is preferably 500 MPa or higher, further, 600 MPa orhigher. The silicon nitride substrate preferably has a fracturetoughness value of 6.0 MPa·m^(1/2) or higher.

The silicon nitride substrate preferably has a thermal conductivity of50 W/m·K or higher. The thermal conductivity is preferably 50 W/m·K orhigher, further 80 W/m·K or higher. Increasing the thermal conductivityallows a thermal resistance of the silicon nitride circuit board to belowered.

The silicon nitride substrate preferably has a thickness of 0.70 mm orsmaller. Further, a thickness reduction down to 0.33 mm or smaller, or0.26 mm or smaller, is preferably performed. The silicon nitridesubstrate is high-strength and excellent in insulation property, whichenables the thickness reduction of the substrate. Performing thethickness reduction of the substrate allows the thermal resistance ofthe circuit board to be lowered. The lower limit of the thickness of thesilicon nitride substrate is preferably 0.10 mm or larger. If thesubstrate has a thickness as small as less than 0.10 mm, it may bedifficult to secure the strength and insulation property of the siliconnitride substrate.

On both sides of the silicon nitride substrate, metal plates areattached. The metal plates are preferably copper plates, aluminumplates, copper alloy plates, or aluminum alloy plates. A method for theattachment may be one in which attachment layers (bonding layers) areinterposed between the silicon nitride substrate and the metal plates,respectively, or one in which the metal plates are directly attached tothe silicon nitride substrate with no attachment layers interposedtherebetween. The method for the attachment using the attachment layersinterposed is preferably an active metal attaching method in which anactive metal brazing material is used.

The active metal brazing material preferably comprises silver (Ag),copper (Cu), and titanium (Ti). Moreover, tin (Sn) and indium (In) aresupposed to be added to the active metal brazing material as necessary.Examples of the active metal brazing material include one that contains40 to 80% by mass of Ag, 20 to 60% by mass of Cu, 0.1 to 10% by mass ofTi, 20% by mass or less (0 included) of Sn, and 20% by mass or less (0included) of In. In a case where the metal plates are copper plates orcopper alloy plates, the attachment is made easier by the active metalattaching method.

In a case where the metal plates are Al plates or Al alloy plates, Al(aluminum) is an active metal. Examples of a component other than theactive metal include Si (silicon). Examples of an active metal brazingmaterial in this case is one that contains 0.01 to 10% by mass of Si,and Al as the remainder.

Assuming that, among the metal plates attached on the front and rearsides, the metal plate on the front side has a thickness of t1, and themetal plate on the rear side has a thickness of t2, |t1−t2|≥0.30 mm isto be satisfied. The satisfaction of |t1−t2|≥0.30 mm indicates that adifference in thickness between the metal plate on the front side andthe metal plate on the rear side is 0.30 mm or larger. Any one of themetal plates on the front side and the rear side may be thicker thananother one. The metal plate on the front side is preferably a metalplate on which a semiconductor element is mounted, and the metal plateon the rear side is preferably a heat dissipation plate.

In order to give an additional current-carrying capacity, the metalplate on the front side is preferably increased in thickness. Incontrast, in order to provide a good heat dissipation property, themetal plate on the rear side is preferably increased in thickness. Inaddition, t1≥0.20 mm and t2≥0.20 mm are preferably established. In orderto satisfy |t1−t2|≥0.30 mm, any one of t1 and t2 is preferably 0.50 mmor larger.

The upper limit of the thicknesses of the metal plates is notparticularly limited. However, it preferably be 5.00 mm or smaller. Athickness of the metal plates exceeding 5.00 mm results in an increasein volume change caused by thermal expansion of the metal plates, whichmakes it difficult to control a warp amount to be described later. Inaddition, 3.00 mm≥|t1−t2|≥0.30 mm is preferably established. Within thisrange, the warp amount to be described later is easy to control.

The silicon nitride substrate warps so that the metal plate attached onone of the front side and the rear side is convex. A feature of thesilicon nitride substrate is that, in this state, warp amounts in bothof a long-side direction and a short-side direction fall within a rangefrom 0.01 to 1.0 mm.

As will be described later, in a case where a leadframe is attached to afront-side metal plate, a convex warp is preferably present on the rearside. In a case where a heat sink or a cooling fin is attached to arear-side metal plate, a convex warp is preferably present on the frontside. In a case where a leadframe is attached to the front-side metalplate, and a heat sink (or a cooling fin) is attached to the rear-sidemetal plate, a convex warp is preferably present on the rear side.

FIG. 1, FIG. 2, and FIG. 3 illustrate an example of the silicon nitridecircuit board according to the embodiment. FIG. 1 and FIG. 3 are topviews, and FIG. 2 is a lateral cross sectional view. In the drawings,reference numeral 1 denotes the silicon nitride circuit board, referencenumeral 2 denotes the silicon nitride substrate, reference numeral 3denotes the metal plate on the front side, and a metal plate 4 denotesthe metal plate on the rear side.

Reference character L1 denotes a length of the silicon nitride substratein a long-side direction (longitudinal width), reference character L2denotes a length of the silicon nitride substrate in a short-sidedirection (transverse width), reference character L3 denotes a length ofthe silicon nitride substrate in a diagonal line direction, andreference character S denotes a warp amount of the silicon nitridesubstrate. A warp amount of the silicon nitride substrate in thelong-side direction is denoted by S_(L1), and a warp amount of thesilicon nitride substrate in the short-side direction is denoted byS_(L2). A warp amount of the silicon nitride substrate in the diagonalline direction is denoted by S_(L3), which is not illustrated.

A method for measuring the warp amount S is as follows. First, astraight line is drawn from one of opposite edges of the silicon nitridesubstrate to another one. The warp amount S is determined as a distanceat which the silicon nitride substrate is most away from the straightline.

In the silicon nitride circuit board according to the embodiment, thewarp amount S_(L1) in the long-side direction and the warp amount S_(L2)in the short-side direction both fall within a range from 0.01 to 1.0mm. The silicon nitride circuit board according to the embodiment hasgiven warp amounts in both of the long-side direction and the short-sidedirection. By adopting such a structure, the silicon nitride circuitboard exerts an excellent TCT property even when a different inthickness between the front and rear metal plates is as large as 0.30 mmor larger. In addition, even when at least one of a leadframe, a heatsink, and a cooling fin having thicknesses of 0.4 mm or larger isattached, the silicon nitride circuit board does not bend, which will bedescribed later. The term “to bend” used herein refers to the warpamount S increasing beyond 1.0 mm. The warp amounts of the siliconnitride substrate in the long-side direction and the short-sidedirection both preferably fall within a range from 0.1 to 1.0 mm.

The warp amount S_(L3) of the silicon nitride substrate in the diagonalline direction preferably falls within a range of 0.1≤S_(L3)≤1.5 mm. Thewarp amount S_(L3) in the diagonal line direction more preferably fallswithin a range of 0.2≤S_(L3)≤0.7 mm.

The leadframe is attached mainly for the purpose of continuity of thesemiconductor element. Therefore, the leadframe is often extended to anoutside of the silicon nitride circuit board. The heat sink and thecooling fin are attached on the rear side of the silicon nitride circuitboard. The heat sink and the cooling fin are attached regularly on therear side. Meanwhile, the leadframe is attached to a spot where acontinuity is intended. Therefore, the leadframe is not always disposedregularly. Setting the warp amount S_(L3) in the diagonal line directionat 0.1 to 1.5 mm makes it possible to set the warp amount of the siliconnitride substrate in an electronic component module at less than 0.1 mm(0 mm included) even when the leadframe is disposed irregularly. Inother words, a silicon nitride circuit board having a warp amount S_(L3)set within a range of 0.1≤S_(L3)≤1.5 mm is suitable for those to whichleadframe are attached. The term “to dispose regularly” refers todispositions of left-right symmetry.

The longitudinal width L1 of the silicon nitride substrate is preferably10 to 200 mm. The transverse width L2 of the silicon nitride substrateis preferably 10 to 200 mm. If the longitudinal width (L1) or thetransverse width (L2) is short, being shorter than 10 mm, a footprint(mounting area) of a semiconductor element on the metal plate on thefront side becomes small, and flexibility of design is decreased. Inaddition, it becomes difficult to form an area in which a bulging-outregion of an attachment layer to be described later is provided. On theother hand, if the longitudinal width (L1) or the transverse width (L2)is long, being longer than 200 mm, it becomes difficult to control thewarp amount (S) so that the warp amount (S) falls within a given range.

A ratio (S_(L1)/S_(L2)) between the warp amount (S_(L1)) in thelong-side direction and the warp amount (S_(L2)) in the short-sidedirection preferably falls within a range from 1.2 to 20.0. In addition,the ratio (S_(L1)/S_(L2)) more preferably falls within a range from 1.2to 6.0. The ratio (S_(L1)/S_(L2)) falling within the range from 1.2 to20.0 indicates that the warp amounts in the long-side direction and theshort-side direction are close to each other. By adopting such astructure, a TCT (thermal cycle test) property can be improved even whenthe difference in thickness between the metal plates on the front andrear sides becomes 0.30 mm or larger, further 0.50 mm or larger.

The metal plate is preferably attached (bonded) to the silicon nitridesubstrate through an attachment layer (bonding layer) containing anactive metal. The active metal refers to one, or two or more of elementsselected from Ti (titanium), Zr (zirconium), Hf (hafnium), Nb (niobium),and Al (aluminum). Among these elements, Ti is preferably used, asmentioned before. The attachment layer containing the active metal ispreferably one in which use is made of an active metal brazing materialthat contains 40 to 80% by mass of Ag, 20 to 60% by mass of Cu, 0.1 to5% by mass of Ti, 20% by mass or less (0 included) of Sn, and 20% bymass or less (0 included) of In. The attachment is made by interposingan active metal brazing material layer between the silicon nitridesubstrate and the metal plate and performing heat treatment. After theattachment, the active metal in the attachment layer is preferablymainly made of an active metal nitride.

The attachment layer preferably bulges out from edge portion of themetal plate. A bulging-out amount of the attachment layer bulging outfrom the edge portion of the metal plate is preferably 30 to 300 μm.FIG. 4 illustrates an example of a cross-sectional view of the siliconnitride circuit board in which the attachment layer includes abulging-out portion. In the drawing, reference numeral 1 denotes thesilicon nitride circuit board, and reference numeral 5 denotes thebulging-out portion of the attachment layer. Reference character Wdenotes the bulging-out amount of the attachment layer. The bulging-outportion 5 may climb up on a side surface of the metal plate. Theprovision of the bulging-out portion enables mitigation of thermalstress on the metal plate and the silicon nitride substrate. Thebulging-out amount W preferably falls within a range from 50 to 300 μm,further a range from 50 to 200 μm.

On the side surface of the metal plate, when a straight line is drawnfrom a point of ½ thickness of the metal plate to an edge portion of themetal plate on a silicon nitride substrate side, an angle between thestraight line and a horizontal line (surface of the substrate) ispreferably 60° or smaller. FIG. 8 illustrates an example of a lateralcross section of the metal plate of the silicon nitride circuit boardaccording to the embodiment. In the drawing, reference numeral 2 denotesthe silicon nitride substrate, reference numeral 3 denotes the metalplate, and reference numeral 5 denotes the bulging-out portion of theattachment layer.

A straight line is drawn from a point of ½ thickness of the metal plate3 to the edge portion of the metal plate. An angle θ between thestraight line and the horizontal line being 60° or smaller indicates ashape of the side surface of the metal plate extending in a directiontoward the silicon nitride substrate side. This shape enables mitigationof thermal stress on the metal plate and the silicon nitride substrate.Therefore, the angle θ between the straight line and the horizontal lineis preferably set to 60° or smaller, further 45° or smaller.

Combination of the bulging-out portion of the attachment layer and theangle θ allows a further mitigation of the thermal stress.

Thus, the TCT (thermal cycle test) property can be improved even whenthe thickness of the metal plate is increased, or the difference inthickness between the metal plates on the front and rear sides (|t1−t2|)is increased. A bulging-out amount W less than 30 μm results in aninsufficient effect of providing the bulging-out portion. A largebulging-out amount W exceeding 300 μm not only results in a failure toobtain an additional effect but also becomes a cause of an increase incost. Therefore, the bulging-out amount W is preferably 30 to 300 μm,further 50 to 200 μm. The angle θ is preferably set to 60° or smaller,further 45° or smaller. This configuration enables the mitigation of thethermal stress, thereby to improve the TCT property.

The metal plate on the rear side is preferably larger in dimension thanthe longitudinal width (L1) and the transverse width (L2) of the siliconnitride substrate. FIG. 5 and FIG. 6 illustrate an example of thesilicon nitride circuit board that has a structure in which the metalplate on the rear side is larger than the silicon nitride substrate.FIG. 5 is a lateral cross sectional view, and FIG. 6 is a top view. Inthe drawings, reference numeral 1 denotes the silicon nitride circuitboard, reference numeral 2 denotes the silicon nitride substrate,reference numeral 3 denotes the metal plate on the front side, andreference numeral 4 denotes the metal plate on the rear side.

Such a structure facilitates increasing the thickness t2 of the metalplate on the rear side. It is possible to establish a relation:t2−t1≥0.3 mm, further t2−t1≥0.8 mm. In addition, the thickness t2 of themetal plate on the rear side can be increased to be 1.0 mm or larger.

Adopting the structure in which the metal plate on the rear side islarger than the silicon nitride substrate allows an efficient emission(radiation) of heat of an electronic component mounted on the metalplate on the front side. In general, the heat generated from theelectronic component performs 45° diffusion (or 45° heat dissipation).Adopting the structure in which the metal plate on the rear side islarger than the silicon nitride substrate allows the 45° diffusion toproceed smoothly.

It is preferable that the metal plate on the front side is a circuitplate, the metal plate on the rear side is a heat dissipation plate, and(a volume of the metal plate on the rear side/a volume of the metalplate on the front side)≥2 is established. This means that a volume ofthe heat dissipation plate (the metal plate on the rear side) is twiceor more as large as a volume of the circuit plate (the metal plate onthe front side). Examples of a method for increasing the volume includeone to increase the thickness of the metal plate on the rear side, andone to increase longitudinal and transverse sizes of the metal plate onthe rear side. In addition, a control of volume ratio between the metalplates on the front and rear sides is also effective in control of thewarp amounts.

As described above, a thermal resistance (Rth) is determined asRth=H/(k×A). Here, H represents heat transfer path, k represents thermalconductivity, and A represents heat dissipation area. Decreasing thethermal resistance (Rth) involves shortening the heat transfer path (H),increasing the thermal conductivity (k), and increasing the heatdissipation area (A). In the silicon nitride circuit board according tothe embodiment, by performing thickness reduction of the silicon nitridesubstrate, it is possible to shorten a heat transfer path of a portion athermal conductivity of which is low. By increasing the thickness of themetal plate or increasing a size of the metal plate, it is possible toincrease the thermal conductivity (k) and the heat dissipation area (A)of the silicon nitride circuit board. Consequently, the thermalresistance (Rth) can be decreased.

The difference in thickness between the metal plates on the front sideand the rear side is provided so that |t1−t2|≥0.30 mm is satisfied. Byincreasing the thickness of the metal plate on the front side, it ispossible to given an additional current-carrying capacity. By increasingthe thickness of the metal plate on the rear side, it is possible toprovide a broad emission path of heat, allowing an improved heatdissipation property. The silicon nitride substrate has warp amounts of0.01 to 1.0 mm in the long-side direction and the short-side direction.Provision of the given warp amounts allows an improved TCT property evenwhen the difference in thickness is large, being 0.30 mm or larger.

There may be a plurality of metal plates on the front side, the metalplates being in a structure in which they are connected to one another.FIG. 7 illustrates an example of the silicon nitride circuit board inwhich a plurality of metal plates are attached on the front side. In thedrawing, reference numeral 1 denotes the silicon nitride circuit board,reference numeral 2 denotes the silicon nitride substrate, referencenumeral 3 denotes the metal plates on the front side, and referencenumeral 4 denotes the metal plate on the rear side. FIG. 7 is aconfiguration example in which two metal plates are arranged on thefront side of the substrate. Also in the structure in which two metalplates are attached on the front side in such a manner, both of themetal plates are to satisfy the relation of |t1−t2|≥0.30 mm. Even in acase where three or more metal plates are attached on the front side,the metal plates are to satisfy |t1−t2|≥0.30 mm.

A silicon nitride circuit board such as the one above is suitable for anelectronic component module on which an electronic component is mounted.Examples of the electronic component include semiconductor elements andthermistors. Since the silicon nitride circuit board according to theembodiment has a decreased thermal resistance, the silicon nitridecircuit board is suitable for a semiconductor element having a highjunction temperature. An SiC device has a high junction temperaturebeing 150° C. or higher. In other words, the circuit board according tothe present invention is suitable for an electronic component module onwhich a SiC device is mounted.

To the metal plate on the front side or the metal plate on the rearside, at least one of a leadframe, a heat sink, and a cooling fin havinga thickness of 0.4 mm or larger is preferably attached. FIG. 9illustrates an example of the electronic component module on which aleadframe is attached. FIG. 10 illustrates an example of the electroniccomponent module on which a heat sink is attached. FIG. 11 illustratesan example of the electronic component module on which a cooling fin isattached. In the drawings, reference numeral 1 denotes the siliconnitride circuit board, reference numeral 6 denotes the electroniccomponent module, reference numeral 7 denotes electronic components,reference numeral 8 denotes the leadframe, reference numeral 9 denotesthe heat sink, and reference numeral 10 denotes the cooling fin.

FIG. 9 illustrates the electronic component module in which theelectronic components 7 and the leadframe 8 are connected to the metalplate 3 on the front side. The leadframe 8 preferably has a thickness of0.4 mm or larger. Examples of the leadframe include a metal plate suchas a copper plate. In addition, by increasing the thickness of theleadframe to 0.4 mm or larger, it is possible to give an additionalcurrent-carrying capacity and give an improved heat dissipationproperty. The thickness of the leadframe has any upper limit, which ishowever preferably 5 mm or smaller.

Adopting such a structure enables a structure that dispenses with wirebonding. By not adopting a wire bonding structure, it is possible toeffectively prevent a wire from being broken in performing resin sealingby a transfer molding method.

In the transfer molding method, an electronic component module isdisposed in a heated mold. The electronic component module is disposedin a space called a cavity. The mold is made up of upper and lower moldsas a set, defining the cavity. Next, a resin tablet (resin in a hardenedform) is charged into a space called a pot. The resin tablet chargedinto the pot starts to melt gradually and is charged into the cavity.With the resin in a liquid state, a space around an electronic componentis filled. When the filling resin is hardened, the electronic componentis brought into a molded state. The transfer molding method allows amolding process to be performed on a lot of electronic component modulesby increasing a size of a mold and thus is a resin sealing methodexcellent in volume productivity (mass productivity).

Meanwhile, the transfer molding method is a manufacturing method inwhich a wire is likely to deform or to be broken. Therefore, by adoptingthe structure dispensing with wire bonding, it is possible to eliminatea break failure or a deformation failure of a wire in performing resinsealing by a transfer molding method.

Although description is made about the advantage of dispensing with wirebonding, continuity of the electronic component may be provided by thewire bonding.

FIG. 10 illustrates the electronic component module in which a heat sink9 is attached to the metal plate on the rear side. The heat sink 9preferably has a thickness of 0.4 mm or larger. The heat sink is made upof a metal plate such as a copper plate. In addition, by increasing thethickness of the heat sink to 0.4 mm or larger, it is possible to givean improved heat dissipation property. The upper limit of the thicknessof the heat sink is not particularly limited, but the thickness ispreferably 10 mm or smaller.

FIG. 11 illustrates the electronic component module in which a coolingfin 10 is attached to the metal plate 4 on the rear side. While the heatsink 9 is in a plate shape, the cooling fin is in a comb shape. The combshape may be in various protrusion shapes such as a groove shape and apin shape. By adopting the comb shape, a heat dissipation area of thecooling fin can be increased. The thickness of the cooling fin does nothave any particular upper limit, but is however preferably 20 mm orsmaller.

Of the leadframe, the heat sink, and the cooling fin, one may beattached, or two may be attached. In the case of attaching two of them,there are a combination of the leadframe and the heat sink, and acombination of the leadframe and the cooling fin.

When the electronic component module according to the embodiment has astructure in which at least one of the leadframe, the heat sink, and thecooling fin having thicknesses of 0.4 mm or larger is attached to themetal plate on the front side or the metal plate on the rear side, thewarp amounts of the silicon nitride substrate in the long-side directionand the short-side direction both can be set at less than 0.1 mm (0 mmincluded).

In conventional practice, when a thick leadframe, heat sink, or coolingfin having a thickness of 0.4 mm or larger is attached, the attachmentincreases warp amounts of a silicon nitride circuit board. In contrast,since the silicon nitride circuit board according to the embodiment hasgiven warp amounts, it is possible to mitigate a stress occurring when athick leadframe or the like is attached. Therefore, the warp amounts ofthe silicon nitride substrate in the long-side direction and theshort-side direction both can be decreased to less than 0.1 mm (0 mmincluded) when the electronic component module is formed.

In addition, decreasing the warp amounts of the silicon nitridesubstrate of the electronic component module facilitates resin sealing.In the case of performing the resin sealing by the transfer moldingmethod mentioned before, when the warp amounts of the silicon nitridesubstrate are small, the melted resin enters gaps stably. As a result,the resin sealing can be performed leaving no gaps. Also from thisregard, the electronic component module according to the embodiment issuitable for a structure on which resin sealing is performed.

If the electronic component module is one to which a heat dissipatingmember such as a leadframe, a heat sink, and a cooling fin is attached,the electronic component module exerts an excellent TCT property. Evenwhen a TCT (thermal cycle test) is actually conducted 600 cycles with 1cycle of the TCT including −45° C.×30 min→room temperature (25° C.)×10min→150° C.×30 min→room temperature (25° C.)×10 min, no peeling occurson the metal plate, and no crack occurs on the silicon nitridesubstrate. Therefore, the electronic component module exerts anexcellent durability even when a SiC device, which has a high junctiontemperature, is used as an electronic component.

Next, a method for producing of the silicon nitride circuit board willbe described. The producing method for the silicon nitride circuit boardaccording to the embodiment does not have any limit, and is any methodas long as the silicon nitride circuit board has the structure mentionedbefore. However, examples of a method to efficiently obtain the siliconnitride circuit board include the following method.

First, a silicon nitride substrate is prepared. The silicon nitridesubstrate is to be one having a three-point flexural strength of 500 MPaor higher. The silicon nitride substrate preferably has a fracturetoughness value of 6.0 MPa·m^(1/2) or higher. In addition, the siliconnitride substrate preferably has a thermal conductivity of 50 W/m·K orhigher. The thermal conductivity is preferably 50 W/m·K or higher,further 80 W/m·K or higher. The silicon nitride substrate preferably hasa thickness of 0.1 mm or larger and 0.70 mm or smaller. Further, athickness reduction down to 0.33 mm or smaller and 0.26 mm or smaller ispreferably performed.

As to the warp amounts of the prepared silicon nitride substrate, thewarp amount S_(L1) in the long-side direction and the warp amount S_(L2)in the short-side direction are both supposed to be less than 0.1 mm.

Next, metal plates are prepared. The metal plates are preferably oneselected from copper plates, copper alloy plates, Al plates, and Alalloy plates. The thicknesses of the metal plates are set so as tosatisfy the relation |t1−t2|≥0.30 mm, where t1 represents the thicknessof the metal plate on the front side, and t2 represents the thickness ofthe metal plate on the rear side.

Next, a step of attaching (bonding) the silicon nitride substrate andthe metal plates is performed. In the attachment step, an active metalbrazing material is used. When the metal plates are copper plates orcopper alloy plates, the active metal is supposed to contain oneselected from Ti (titanium), Zr (zirconium), Hf (hafnium), and Nb(niobium). As the active metal, Ti is the most preferable. Componentsother than the active metal may include Ag, Cu, In, and Sn. As theactive metal brazing material, one that contains 40 to 80% by mass ofAg, 20 to 60% by mass of Cu, 0.1 to 10% by mass of Ti, 20% by mass orless (0 included) of Sn, and 20% by mass or less (0 included) of In.

In a case where the metal plates are Al plates or Al alloy plates, Al(aluminum) is used as an active metal. Examples of a component otherthan the active metal include Si (silicon). As an active metal brazingmaterial in this case, use is made of a brazing material that contains0.01 to 10% by mass of Si, and Al as the remainder.

The side surface of the metal plate may be worked in advance so that,when a straight line is drawn from a point of ½ thickness of the metalplate to an edge portion of the metal plate on a silicon nitridesubstrate side, an angle between the straight line and a horizontal lineis 60° or smaller. In addition, the metal plate may be treated byetching treatment after the attachment so that, when a straight line isdrawn from a point of ½ thickness of the metal plate to an edge portionof the metal plate on a silicon nitride substrate side, an angle betweenthe straight line and the horizontal line is 60° or smaller.

A resin binder is added to the active metal brazing material, whereby anactive metal brazing material paste is prepared. The active metalbrazing material paste is applied to the silicon nitride substrate,whereby an active metal brazing material paste layer is formed. On theactive metal brazing material paste layer, the metal plate is disposed.In the case of providing a bulging-out portion of an attachment layer,the active metal brazing material paste layer is provided as one that isslightly larger than a size of the metal plate in the longitudinal andtransverse directions. An application thickness of the active metalbrazing material paste layer preferably falls within a range from 10 to40 μm. A thickness of the active metal brazing material paste layer lessthan 10 μm leads to a risk that a sufficient attachment strength(bonding strength) is not obtained. A large thickness of the activemetal brazing material paste layer exceeding 40 μm not only results in afailure to obtain a further improved attachment strength but alsobecomes a cause of an increase in cost. Therefore, the thickness of theactive metal brazing material paste layer preferably falls within arange from 10 to 40 μm, further 15 to 25 μm.

Next, a heating step is performed. A heating temperature preferablyfalls within a range from 600 to 900° C. In a case where the activemetal brazing material contains one selected from Ti, Zr, Hf, and Nb, anattachment temperature preferably falls within a range from 750 to 900°C. In a case where the active metal brazing material contains Al, theattachment temperature preferably falls within a range from 600 to 750°C. The heating step is preferably performed in a vacuum atmosphere. Adegree of vacuum is preferably 1×10⁻² Pa or lower, further 4×10⁻³ Pa orlower. By performing the heating step in the vacuum atmosphere, it ispossible to effectively prevent the copper plate or the active metalbrazing material from being oxidized or nitrided. A cooling rate afterthe heating step is preferably set at 80° C./h or lower. By setting thecooling rate at a low rate, it is possible to mitigate thermal stress inthe heating step.

By performing such steps, it is possible to manufacture a siliconnitride circuit board having a warp amount S_(L1) in the long-sidedirection and a warp amount S_(L2) in the short-side direction that fallwithin a range from 0.01 to 1.0 mm.

As necessary, a step of attaching one or two of a leadframe, a heatsink, and a cooling fin is performed. The leadframe, the heat sink, andthe cooling fin are preferably those having thicknesses of 0.4 mm orlarger. The attachment of a heat dissipating member such as theleadframe, the heat sink, and the cooling fin is performed using solder.

On the metal plate on the front side, an electronic component ismounted. Examples of the electronic component may include semiconductorelements and thermistors. The mounting of the electronic component ispreferably performed using solder. The attachment of the heatdissipating member such as the leadframe and the attachment of theelectronic component may be performed at a same time in a same step ormay be performed in different steps.

As necessary, a resin sealing step is performed. The resin sealing stepis preferably performed by a transfer molding method. The transfermolding method is a method excellent in mass productivity. The siliconnitride circuit board according to the embodiment allows the warpamounts to be decreased even when the resin sealing is performed by thetransfer molding method.

EXAMPLES Examples 1 to 7 and Comparative Examples 1 to 3

As a silicon nitride substrate, a substrate having a thickness of 0.32mm, a long-side direction length (L1) of 50 mm, a short-side directionlength (L2) of 35 mm was prepared. The silicon nitride substrate had athree-point flexural strength of 600 MPa, a thermal conductivity of 90W/m·K, and a fracture toughness value of 6.5 MPa·m^(1/2). Warp amountsof the silicon nitride substrate before attachment of copper plates werea warp amount S_(L1) in a long-side direction=0.02 mm, a warp amountS_(L2) in a short-side direction=0.01 mm, and a warp amount S_(L3) in adiagonal line direction=0.01 mm.

Next, as metal plates (circuit plate), copper plates were prepared. Asan active metal brazing material, a brazing material containing Ag (60wt %), Cu (30 wt %), In (8 wt %), and Ti (2 wt %) was prepared. Thebrazing material and a resin binder were mixed together, whereby anactive metal brazing material paste was prepared. To both surfaces ofthe silicon nitride substrate, the active metal brazing material pastewas applied, the copper plates were disposed, and the heating step wasperformed in a vacuum atmosphere, at 1×10⁻³ Pa or lower, at 780 to 830°C. A cooling rate after the heating step was set at 50 to 70° C./h forExamples and at 300 to 400° C./h for Comparative Examples.

Sizes of the copper plates on the front side, sizes of the copper plateson the rear side, side surface angles of the metal plates, andbulging-out amounts of the active metal brazing material are shown inTable 1. The side surface angles of the metal plate were each determinedas an angle θ of a straight line drawn from a point of ½ thickness ofthe metal plate to an edge portion of the metal plate on the siliconnitride substrate side, as illustrated in FIG. 8.

TABLE 1 Sizes of Copper Plate Sizes of Copper Plate Side SurfaceBulging-out Amount on Front Side on Rear Side Angle of Metal of ActiveMetal (Length × Width × (Length × Width × Plate Brazing Material SampleNo. Thickness mm) Thickness mm) θ (°) (μm) Example 1 40 × 25 × 0.3 40 ×30 × 0.6 60 50 Example 2 40 × 25 × 0.3 40 × 30 × 1.0 40 70 Example 3 40× 25 × 0.3 40 × 30 × 2.0 50 80 Example 4 30 × 30 × 0.8 40 × 30 × 3.0 60100 Example 5 35 × 30 × 0.4 60 × 45 × 5.0 45 120 Example 6 20 × 20 ×0.3, 60 × 45 × 7.0 40 150 20 × 20 × 0.4 Example 7 40 × 25 × 0.4 40 × 30× 1.5 50 200 Comparative 40 × 25 × 0.3 40 × 25 × 0.3 45 0 Example 1Comparative 40 × 25 × 0.25 40 × 25 × 0.50 45 50 Example 2 Comparative 40× 25 × 0.25 40 × 25 × 0.50 90 200 Example 3

For the silicon nitride circuit boards obtained by the producing methoddescribed above, the warp amount S_(L1) in the long-side direction, thewarp amount S_(L2) in the short-side direction, the warp amount S_(L3)in the diagonal line direction, and a direction of a warp were measured.The measurement of the warp amount was performed by drawing a straightline from one of opposite edges of the silicon nitride substrate toanother one, as shown in FIG. 2, and determining the warp amount as adistance at which the silicon nitride substrate is most away from thestraight line. Results of the measurement are shown in Table 2 below.

TABLE 2 Warp Amount in Warp Amount in Warp Amount in Long Side ShortSide Diagonal Line Direction Direction Ratio Direction Sample No. S_(L1)(mm) S_(L2) (mm) (S_(L1)/S_(L2)) S_(L3) (mm) Warp Direction Example 10.33 0.15 2.2 0.27 Convex toward Rear Side Example 2 0.8 0.5 1.6 0.7Convex toward Front Side Example 3 0.7 0.6 1.2 0.55 Convex toward FrontSide Example 4 1.0 0.5 2.0 0.9 Convex toward Rear Side Example 5 0.8 0.61.3 0.7 Convex toward Rear Side Example 6 0.6 0.1 6.0 0.5 Convex towardRear Side Example 7 0.55 0.4 1.4 0.45 Convex toward Rear SideComparative 0.00 0.00 — — — Example 1 Comparative 0.25 0.25 1.0 0.20Convex toward Example 2 Rear Side Comparative 0.4 0.3 1.3 0.3 Convextoward Example 3 Rear Side

The silicon nitride circuit boards in Examples had warp amounts intendedas an objective.

Examples 8 to 17

As silicon nitride substrates, substrates shown in Table 3 wereprepared. As to warp amounts of the substrates, warp amounts in thelong-side direction were 0.02 mm or smaller, and warp amounts in theshort-side direction were 0.02 mm or smaller. The step of attachingmetal plates was the same as that in Example 1.

TABLE 3 Three- Point Fracture Sizes of Substarte Flexural ThermalToughness (Length × Width × Strength Conductivity value Sample No.Thickness mm) (MPa) (W/m · K) (MPa · m^(1/2)) Example 8 200 × 100 × 0.32700 50 7.2 Example 9 100 × 80 × 0.32 650 60 6.0 Example 10 30 × 20 ×0.25 650 90 6.3 Example 11 60 × 40 × 0.25 550 60 6.5 Example 12 55 × 50× 0.25 600 90 6.8 Example 13 70 × 50 × 0.25 620 90 6.8 Example 14 60 ×40 × 0.20 670 95 6.7 Example 15 55 × 40 × 0.50 600 90 6.8 Example 16 55× 30 × 0.635 600 90 6.8 Example 17 50 × 40 × 0.635 650 90 6.6

Next, copper plates shown in Table 4 were attached.

TABLE 4 Sizes of Copper Bulging-out Sizes of Copper Plate Plate Amountof on Front Side on Rear Side Side Surface Angle Active Metal (Length ×Width × (Length × Width × of Metal Plate Brazing Sample No. Thicknessmm) Thickness mm) θ (°) Material (μm) Example 8 Three Pieces of 180 × 80× 1.0 45 80 30 × 30 × 0.3 Example 9 40 × 25 × 0.3, 104 × 84 × 2.0 35 10020 × 20 × 0.5 Example 10 20 × 15 × 0.3 40 × 30 × 2.0 60 120 Example 1140 × 30 × 0.3 50 × 30 × 2.0 50 100 Example 12 35 × 30 × 0.4 60 × 45 ×5.0 50 120 Example 13 20 × 20 × 0.3, 60 × 45 × 7.0 45 70 20 × 20 × 0.4Example 14 40 × 20 × 0.3 40 × 30 × 0.9 40 20 Example 15 Two Pieces of 45× 30 × 1.0 45 30 20 × 20 × 0.2 Example 16 40 × 20 × 0.4 45 × 20 × 1.5 4525 Example 17 Two Pieces of 45 × 35 × 1.6 45 40 30 × 15 × 0.3

For the obtained silicon nitride circuit boards, the warp amount S_(L1)in the long-side direction, the warp amount S_(L2) in the short-sidedirection, the warp amount S_(L3) in the diagonal line direction, and adirection of a warp were measured. The measurement of the warp amountwas performed by drawing a straight line from one of opposite edges ofthe silicon nitride substrate to another one, as shown in FIG. 2, anddetermining the warp amount as a distance at which the substrate is mostaway from the straight line. Results of the measurement are shown inTable 5 below.

TABLE 5 Warp Amount in Warp Amount in Warp Amount in Long Side ShortSide Diagonal Line Direction Direction Ratio Direction Sample No. S_(L1)(mm) S_(L2) (mm) (S_(L1)/S_(L2)) S_(L3) (mm) Warp Direction Example 80.9 0.65 1.4 1.0 Convex toward Rear Side Example 9 0.7 0.2 3.5 0.7Convex toward Rear Side Example 10 0.3 0.1 3.0 0.2 Convex toward RearSide Example 11 0.4 0.15 2.7 0.35 Convex toward Front Side Example 120.5 0.2 2.5 0.5 Convex toward Rear Side Example 13 0.85 0.65 1.3 0.9Convex toward Rear Side Example 14 1.0 0.2 5.0 0.7 Convex toward RearSide Example 15 0.9 0.1 9.0 0.7 Convex toward Rear Side Example 16 0.150.01 15.0 0.1 Convex toward Rear Side Example 17 0.9 0.05 18.0 0.95Convex toward Rear Side

Next, in Examples 1 to 17 and Comparative Examples 1 to 3, TCTproperties of the silicon nitride circuit boards and attachmentstrengths of the metal plates were measured. The TCT (thermal cycletest) was conducted with 1 cycle thereof including −40° C.×30 min ofretention→room temperature (25° C.)×10 min→125° C.×30 min→roomtemperature (25° C.)×10 min, and presence/absence of peeling in themetal plate and occurrence of a crack in the silicon nitride substrateafter 600 cycles were measured. The attachment strengths of the metalplates were measured in terms of peel strength. Results of themeasurement are shown in Table 6.

TABLE 6 TCT (Presence/Absence of Peeling in Metal Plate and AttachmentStrength Occurrence of Crack in of Metal Plate Sample No. SiliconNitride Substrate) (kN/m) Example 1 None 25 Example 2 None 23 Example 3None 25 Example 4 None 28 Example 5 None 25 Example 6 None 30 Example 7None 25 Comparative None 20 Example 1 Comparative None 25 Example 2Comparative None 28 Example 3 Example 8 None 25 Example 9 None 23Example 10 None 28 Example 11 None 30 Example 12 None 22 Example 13 None26 Example 14 None 26 Example 15 None 27 Example 16 None 25 Example 17None 25

As is clear from the results shown in Table 6 above, the silicon nitridecircuit boards in Examples and Comparative Examples were excellent inTCT property and attachment strength.

Next, a leadframe, a heat sink, a cooling fin were attached. Theleadframe, the heat sink, and the cooling fin were all manufacturedusing copper plates. As an electronic component, a semiconductor elementwas mounted on a front-side copper plate. The attachment of a heatdissipating member such as the leadframe, the heat sink, and the coolingfin, and the mounting of the semiconductor element were performed bysolder attachment. Through these steps, electronic component moduleswere manufactured.

For the obtained electronic component modules, warp amounts and TCTproperties of the silicon nitride substrates were measured. The warpamounts of the silicon nitride substrates were measured by the samemethod as that in Example 1. As to the TCT properties, the TCT (thermalcycle test) was conducted with 1 cycle including −45° C.×30 min→roomtemperature (25° C.)×10 min→150° C.×30 min of retention→room temperature(25° C.)×10 min, presence/absence of peeling in the metal plate andoccurrence of a crack after 600 cycles and 1000 cycles were performedwere investigated. Results of the measurement and investigation areshown in Table 7 and Table 8 below.

TABLE 7 TCT (Presence/Absence of Peeling in Metal Plate Warp Amounts ofand Silicon Niotride Substrate Occurrence of Crack in Warp Amount inWarp Amount of Silicon Nitride Heat Dissipation Member Long-SideShort-Side Substrate) (Length × Width × Thickness Direction: S_(L1)Direction: S_(L2) 1000 Sample No. mm) (mm) (mm) 600 Cycles CyclesExample 1 Leadframe 0.07 0.02 None None (50 × 10 × 0.4) Example 2 HeatSink 0.05 0.04 None None (50 × 40 × 2.0) Example 3 Cooling Fin 0.08 0.06None Observed (50 × 40 × 3.0) Example 4 Leadframe 0.07 0.03 None None(30 × 10 × 1.0) Example 5 Leadframe 0.09 0.05 None None (30 × 10 × 0.8)Example 6 Each of Leadframes are 0.07 0.07 None None attached (30 × 10 ×0.6) Example 7 Leadframe 0.06 0.04 None None (20 × 10 × 0.5) ComparativeLeadframe 0.25 0.15 Observed — Example 1 (20 × 10 × 0.5) ComparativeLeadframe 0.08 0.06 Observed — Example 2 (20 × 10 × 0.5) ComparativeLeadframe 0.07 0.03 Observed — Example 3 (20 × 10 × 0.5)

TABLE 8 Warp Amounts of Silicon Nitride Substrate TCT Warp Amount(Presence/Absence of Warp Amount in Short- Peeling in Metal Plate andHeat Dissipation Member in Long-Side Side Occurrence of Crack in (Length× Width × Thickness Direction Direction Silicon Nitride Substrate)Sample No. mm) S_(L1) (mm) S_(L2) (mm) 600 Cycles 1000 Cycles Example 8Leadframes were connected 0.08 0.06 None None (20 × 8 × 0.5) Example 9Leadframes were connected 0.08 0.08 None None (20 × 10 × 0.5) Example 10Leadframes were connected 0.05 0.05 None Observed (20 × 10 × 1.0)Cooling Fin was connected (44 × 34 × 2.0) Example 11 Heat Sink wasconnected 0.08 0.07 None None (45 × 35 × 2.0) Example 12 Leadframe wasconnected 0.05 0.05 None None (20 × 10 × 0.6) Heat Sink was connected(62 × 46 × 1.0) Example 13 Leadframes were connected 0.06 0.04 None None(20 × 10 × 0.5) Example 14 Leadframe was connected 0.09 0.06 None None(30 × 10 × 0.5) Example 15 Leadframes were connected 0.03 0.02 None None(30 × 10 × 0.6) Example 16 Leadframe was connected 0.02 0.02 None None(30 × 10 × 0.5) Example 17 Leadframes were connected 0.02 0.02 None None(30 × 10 × 0.6)

The electronic component module in each Example exerted excellent TCTproperties even when the heat dissipating member (the leadframe, theheat sink, and the cooling fin) having a thickness of 0.4 mm or largerwas attached. In particular, even when the TCT test with ahigh-temperature region set at a higher temperature of 150° C. wasconducted, good results were also obtained.

In contrast, as to the silicon nitride circuit board in ComparativeExample 1, although the warp amounts were small, the warp amounts wereincreased when the leadframe was attached. As a result, the TCT propertywas decreased as an electronic component module.

Since the electronic component module in each Example allows continuityusing a leadframe, a structure dispensing with wire bonding can beadopted. Therefore, occurrence of a deformation failure or a breakfailure of a wire is avoided when the resin sealing is performed by thetransfer molding method. In other words, the electronic component moduleaccording to the embodiment is suitable for a resin sealed structure bythe transfer molding method.

Several embodiments of the present invention are illustrated above.However, these embodiments are presented by way of example only and notintended to limit the scope of the invention. These novel embodimentscan be implemented in other various forms. Various omissions,substitutions, alterations, and the like of the embodiments can be madewithout departing from the gist of the invention. These embodiments andmodifications thereof are included in the scope and the gist of theinvention and included in the inventions described in claims and a scopeof equivalents of the inventions. The embodiments described above can beimplemented in combination with one another.

REFERENCE SIGNS LIST

-   1 silicon nitride circuit board-   2 silicon nitride substrate-   3 metal plate on front side-   4 metal plate on rear side-   5 bulging-out portion of attachment layer-   6 electronic component module-   7 electronic component-   8 leadframe-   9 heat sink-   10 cooling fin

The invention claimed is:
 1. A silicon nitride circuit board in whichmetal plates are attached on front and rear sides of a silicon nitridesubstrate having a three-point flexural strength of 500 MPa or higher,wherein assuming that a thickness of the metal plate on the front sideis denoted by t1, and a thickness of the metal plate on the rear side isdenoted by t2, a numerical relation: |t1−t2|≥0.30 mm is satisfied, and awarp is formed in the silicon nitride substrate so that the siliconnitride substrate is convex toward the metal plate on one of the frontside or the rear side; and warp amounts of the silicon nitride substratein a long-side direction and a short-side direction both fall within arange from 0.01 to 1.0 mm; wherein assuming that a warp amount of thesilicon nitride substrate in the long-side direction is denoted byS_(L1), and a warp amount of the silicon nitride substrate in theshort-side direction is denoted by S_(L2), a ratio (S_(L1)/S_(L2)) fallswithin a range from 1.2 to 20.0; and wherein assuming that a length ofthe silicon nitride substrate in a diagonal line direction is denoted byL3, and a warp amount in the diagonal line direction is denoted byS_(L3), S_(L3) falls within a range of 0.1≤S_(L3)≤1.5 mm.
 2. The siliconnitride circuit board according to claim 1, wherein a longitudinal width(L1) of the silicon nitride substrate falls within a range from 10 to200 mm, and a transverse width (L2) of the silicon nitride substratefalls within a range from 10 to 200 mm.
 3. The silicon nitride circuitboard according to claim 1, wherein on a side surface of the metalplate, when a straight line is drawn from a point of ½ thickness of themetal plate to an edge portion of the metal plate on a silicon nitridesubstrate side, the straight line and a surface of the silicon nitridesubstrate forms an angle of 60° or smaller.
 4. The silicon nitridecircuit board according to claim 1, wherein each of the metal plates isattached to the silicon nitride substrate through an attachment layercontaining an active metal interposed therebetween, and a bulging-outamount of the attachment layer bulging out from an edge portion of themetal plate is 30 to 300 μm.
 5. The silicon nitride circuit boardaccording to claim 1, wherein the metal plate on the rear side is largerthan a longitudinal width (L1) and a transverse width (L2) of thesilicon nitride substrate.
 6. The silicon nitride circuit boardaccording to claim 1, wherein the silicon nitride substrate has athermal conductivity of 50 W/m·K or higher and a thickness of 0.70 mm orsmaller.
 7. The silicon nitride circuit board according to claim 1,wherein the metal plate on the front side is a circuit plate, the metalplate on the rear side is a heat dissipation plate, and (a volume of themetal plate on the rear side/a volume of the metal plate on the frontside)≥2 is established.
 8. The silicon nitride circuit board accordingto claim 1, wherein there are a plurality of metal plates attached onthe front side.
 9. An electronic component module comprising the siliconnitride circuit board according to claim 1, on which an electroniccomponent is mounted.
 10. The electronic component module according toclaim 9, wherein at least one of a leadframe, a heat sink, and a coolingfin having thicknesses of 0.4 mm or larger is attached to the metalplate on the front side or the metal plate on the rear side.
 11. Theelectronic component module according to claim 9, wherein at least oneof a leadframe, a heat sink, and a cooling fin having thicknesses of 0.4mm or larger is attached to the metal plate on the front side or themetal plate on the rear side, and a warp amount of the silicon nitridesubstrate in the long-side direction and a warp amount of the siliconnitride substrate in the short-side direction are both less than 0.1 mm(0 mm included).
 12. The electronic component module according to claim9, wherein the electronic component module is subjected to resin sealingby a transfer molding method.
 13. The electronic component moduleaccording to claim 9, wherein continuity with the electronic componentdispenses with wire bonding.